1. Field of the Invention
The present invention relates to a wireless communication receiver (which will be simply referred to as a “receiver” hereinafter) and, more particularly, to a direct conversion type receiver using programmable gain amplifiers by which gains are adjustable in steps and being able to suppress the effect of noise that accompanies gain change by the programmable gain amplifiers.
2. Description of the Related Art
One type of signals to be received by receivers for Wideband Code Division Multiple Access (W-CDMA), standardization of which is pursued by the 3rd Generation Partnership Project (3GPP), Dedicated Physical Channel signals (which will be referred to as DPCH signals hereinafter) are formatted in a recommended format which is shown in FIG. 2A.
The DPCH signals are transmitted and received in frames, each frame being 10 msec., and one frame is divided into 15 slots, each slot being 666 μsec. One slot contains two data signals Data 1 and Data 2 and three control signals TPC, TFCI, and Pilot.
The TPC (Transmit Power Control) signal is for controlling electric power of transmission to a base station. The TFCI (Transport Format Combination Indicator) signal is for identifying the format type per frame that differs, depending on the transfer rate. The pilot signal is for detecting a phase and keeping synchronization with a base station.
Powerful error correction using turbo codes or the like does not apply to these control signals. As measures to protect the signals, transmission power is adjusted so that the power of the control signals only can be increased. One slot is composed of Data 1, TPC, TFCI, Data2, and Pilot parts which are sequenced in this order and time to be taken to receive these signals differs, depending on what slot format being received.
As methods of frequency conversion from a Radio Frequency (RF) band to a baseband in a receiver application, superheterodyne and direct downconversion are generally known.
Superheterodyne is a frequency conversion method in which the frequency of received signals in an RF band is downconverted twice to a baseband. That is, the frequency of received signals is first converted from the RF band to an intermediate frequency (IF) band and then converted to the baseband.
Direct conversion is a frequency conversion method in which the frequency of received signals is decreased to a baseband by downconverting the signals once.
As an example of receivers using direct conversion (which will be referred to as a “direct conversion receiver” hereinafter), we refer to “A 22 mA 3.7 dB NF Direct Conversion Receiver for 3G WCDMA” in an ISSCC Digest of Technical Papers, Feb. 5–7, 2001, San Francisco, USA, pp. 284–285.
The circuitry arrangement of this direct conversion received will now be described, using FIG. 3. Signals received from an antenna are amplified by a low noise amplifier 301 and distributed to two paths. In either path, the DC component of a signal is cut off by a capacitor 302 and its frequency is downconverted to a baseband by a mixer 303. During the downconversion, each mixer 303 multiples each of the two signals carried through the two paths by a local signal whose phase differs by 90 degrees for each signal, thereby generating one signal having a sine (sin) component and the other signal having a cosine (cos) component which intersect at right angles. A signal input at a terminal LOC is supplied as the local signal after passing through a Resistance Capacitance-Poly Phase (RC-PP) filter. Then, a low-pass filter 304 removes the adjacent channel signal power from the frequency-downconverted signal and outputs the signal with required gain. Next, a programmable gain amplifier 305 adjusts the gain of the signal so that its voltage level always falls within the dynamic range of an analog-to-digital (A/D) converter 306.
Moreover, as a document that focuses on the analog baseband portion of a direct conversion receiver, we refer to M. Goldfarb et al., “ANALOG BASEBAND IC FOR USE IN DIRECT CONVERSION W-CDMA RECEIVERS” in a 2000 IEEE Radio Frequency Integrated Circuits Symposium, pp. 79–89.
The receiving power of a mobile communications device varies, depending on the time when it receives and the place where it is. To maintain the voltage of received signals at a constant level when being input to the A/D converter, the programmable gain amplifier is necessary in the analog portion of the receiver. In the direct conversion, the programmable gain amplifier compensates for an insufficient gain unachievable by the low noise amplifier stage before the mixer or an excessively amplified gain. Because of a problem such as distortion by the mixer, the programmable gain amplifier is normally used at a stage behind the mixer.
Kokai (Japanese Unexamined Patent Publication) No. 2001-36362 discloses a programmable gain amplifier arrangement in receiver circuitry, in which cascode amplifiers and attenuators are assembled into it in order to extend the range of controllable gains.
Kokai No. 2001-36367, Kokai No. 2001-44776, and Kokai No. 2001-53564 disclose programmable gain amplifier arrangements including an attenuator circuit at the input stage thereof in order to suppress noise and distortion.
Moreover, as concerns how to control programmable gain amplifiers, Kokai No. 2001-111523 discloses controlling the timing at which a programmable gain amplifier by which gains are adjustable continuously starts gain control by means of calculation of received signal power. Kokai No. Hei-5-335857 (No. 335857 of 1993) discloses control causing programmable gain amplifiers to decrease the gain if synchronization is not attained.